Hi all I have to analyze failures of a subassemblies of our subcontractor. As cause of problem I suppose cracks/voids in the prepreg (of a16-layer pcb), caused either by poor siloxane-treatment between glass and epoxide - or - because crushing of prepreg integrity by microlocal mechanical stress during the pcb press operation. As cause of this stress I assume following: During the press operation the prepreg conforms to the bounds formed by the laminate and their conductors at both sides. Between the walls/shoulders ot two buried vias the prepreg will be pressed, the resin will flow away, but the glass will be squeezed. I think glass fibres squeezing together can resulte to resin recession. But let me tell you some details: The failing subassemblies I've to analyze, contains transformer windings which are implemented as a 16-layer pcb. The winding shows dielectric breakdown between different winding layers (and sometimes between different windings on the same layer). This breakdown happens after some hours run-in at 120°C and 500VDC (500VDC as test voltage, failures also at lower tension at at AC) .Before them, all pcb has passed successful a hi-voltage test at ambient temperature. The diel. strength for the closest distance of 65mm prepreg should be around 2.5kV, or a little bit less because IPC-2221, § 4.1.2, not 500V! I tend to see any loss of the electric strenght of a laminate/prepreg as function of the temperature, the hi-temp aging, and material defects. The occuring pcb temperature of 120°C seems in accordance with IPC-2222, § 4.3. So I'm looking for material defects, especially cracks: if cracks span a part of the 65mm (prepreg), the electric strenght will decrease, because the electric strenght of a cracks (like air) amounts 1kV/mm (instead of 40kV/mm. Hope to get some engelnotes. Bernhard ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################